The present invention relates to a dispersion having a modified carbon black dispersed therein, and a water-based ink containing the dispersion, which can be used for a recording liquid, in a writing instrument, and so on.
Carbon black is widely used as a colorant in water-based inks for printing a black color with ink jet printers.
In general a carbon black raw material powder comprises a secondary agglomerated mass called an agglomerate formed through van der Waals attraction or mere congregation, adhesion or interlocking between aggregates that are primary aggregated bodies in each of which approximately spherical primary particles are congregated together. In an untreated state, such a carbon black raw material powder will not disperse in water but rather will settle rapidly.
A water-based ink containing such a carbon black raw material powder and a dispersant for dispersing the carbon black raw material powder in water has high viscosity, and hence printing is difficult. Modified carbon blacks obtained by oxidizing the surface of the carbon black raw material powder to introduce hydrophilic functional groups such as carboxyl groups so that dispersion of the carbon black raw material powder in water becomes easy even if a dispersant is not used are thus used.
As such a modified carbon black, in Patent Document 1 the present applicants have disclosed one for which the specific surface area and the DBP oil absorption (the amount of dibutyl phthalate absorbed by 100 g of the carbon black) are specified.
(Patent Document 1) Japanese Patent Application Laid-open No. 2000-319573
Water-based inks containing this modified carbon black have a low viscosity, and moreover have a high print density, and hence dense printing with a deep black color is possible. When using such a water-based ink as an ink jet recording ink, the water-based ink must have sufficient stability over time for practical use. Water-based inks hitherto have not always had sufficient storage stability, with the viscosity increasing, or the carbon black particles further agglomerating and hence settling becoming prone to occurring, after a long period has elapsed. There have thus been calls for water-based inks for which the settling rate is yet lower and clogging is yet less prone to occurring.